It has been known for years that ventricular fibrillation, a fatal arrhythmia, can be reversed by passing high energy electric current through the fibrillating myocardium. In a hospital coronary care environment, defibrillation is generally accomplished by means of external chest paddles placed on the patient's thorax, with current being diffused through the chest. While defibrillation is in this manner generally successful, only a portion of the applied current affects the myocardium, and hence substantial amounts of electrical energy must be introduced to the already suffering body of the patient.
In open heart surgery, internal paddles are commonly applied directly to the surface of the heart. Typically, such endothoracic paddles are circular in configuration, of a conductive metal such as stainless steel, and are approximately 10 cm. in diameter. In use, these paddles are generally applied to opposite surfaces of the ventricular myocardium in a sandwich-type fashion.
A modern approach to defibrillation considers the use of a single intravascular catheter electrode system having two discrete electrodes (or electrode sets) on the catheter. In catheter defibrillation, the electrical current travels from one electrode to the other, setting up an electrical field which affects a critical mass of the myocardium. By so depolarizing this critical mass, the heart is brought back to normal cardiac rhythm.
In humans, external paddle defibrillation is known to require from 100 to 400 watt-seconds of energy, while endothoracic paddle defibrillation employed during heart surgery generally requires the application of energy somewhere on the order of 10 to 50 watt-seconds. The approach of catheter defibrillation has reduced the energy requirements to somewhere between 5 and 35 watt-seconds.
The utilization of an implantable automatic defibrillator implies an elective installation of the necessary electrodes, for if fibrillation is in progress, there is no time for the installation of internal electrodes. Being an elective procedure, where the election to implant electrodes is based upon the statistical probability of the occurrence of ventricular fibrillation, it is clear that the acceptability of a given electrode system will be a function of how easily the electrodes can be installed, assuming, of course, acceptable performance. If a catheter electrode system is used, the installation consists of the relatively simple placement pervenously of a bielectrode catheter the tip of which goes into the apex of the right ventricle. Yet there is a need for defibrillating electrodes which more efficiently discharge energy into the heart.
There are an extremely large number of coronary bypass operations done yearly (approximatley 50,000) on patients with coronary artery disease. For the most part, these patients are at a high risk of ventricular fibrillation and other potentially fatal arrhythmias, both during the post-operative phase and on a long term basis. Immediately after the coronary artery has been bypassed, but before the chest is closed, there is an opportunity to install electrodes for accomplishing ventricular defibrillation. At this time, a set of electrodes, if available, could be laid into position on the heart surface.
The heart is normally covered with a pericardial membrane or sac. During coronary bypass surgery, the pericardium is incised and laid open to gain access to the coronary arteries, enabling the implantation of electrodes between the pericardium and the epicardial surface of the heart. Or, if the pericardium were to be partially destroyed during surgery, the chest would still be open to enable the placement of suitable electrodes at another location. For example, one or both of the electrodes could be sutured to the outside of the pericardium or other remaining structures. Accordingly, there is a need for defibrillating, or more generally, cardioverting electrodes which are suitable for implantation on or about the heart during open heart surgery, such as coronary bypass surgery.
Furthermore, being an elective procedure, it is important that the implantation of cardioverting electrodes be made possible at minimum risk to the patient. With the exception of the catheter, there are no known cardioverting electrodes which can be implanted without entering the pleural space, necessitating a general anesthetic and respiration assistance. There is accordingly a great need for cardioverting electrodes which may be implanted through the means of a relatively simple surgical procedure.
While defibrillator technology has advanced significantly since its inception, it may still be thought of as in its infancy. This is especially true with respect to the automatic implantable defibrillator. The present invention adds one further dimension to medical electronics, and in particular to the field of low energy reliable automatic implantable defibrillation.